Lithium niobate (LiNbO3: hereinafter, abbreviated as LN) single crystals are artificial ferroelectric crystals having a melting point of approximately 1250° C. and a Curie temperature of approximately 1140° C. In addition, a lithium niobate single crystal substrate (hereinafter, referred to as an LN substrate) made of LN single crystals is utilized as a material for a surface acoustic wave device (SAW filter) used for removing noises from electrical signals used mainly in mobile communications devices.
The SAW filter has a structure in which a comb electrode is formed of a metal thin film of an AlCu alloy or the like on a substrate formed of a piezoelectric material such as an LN single crystal. This comb electrode plays an important role that affects the properties of the device. In addition, the comb electrode is formed by first forming a metal thin film on a piezoelectric material by a sputtering method or the like, and then etching and removing an unnecessary portion while leaving a comb pattern by a photolithographic technique.
In addition, the LN single crystal, serving as the material for the SAW filter, is industrially obtained by the Czochralski process, in which the LN single crystal is generally grown in an electric furnace having an atmosphere of a nitrogen-oxygen mixed gas in which the concentration of oxygen is approximately 20% by using a platinum crucible, and then cooled at a predetermined cooling rate in the electric furnace, and thereafter taken out of the electric furnace.
The LN single crystal thus grown is colorless and transparent or takes on a pale yellow color with a high transparency. After the growth, to remove residual strain due to the thermal stress during the growth, the LN single crystal is subjected to heat treatment under soaking at a temperature close to its melting point, and is subjected to poling treatment for obtaining single polarity, that is, to a series of treatments in which the temperature of the LN single crystal is increased from room temperature to a predetermined temperature of the Curie temperature or more, a voltage is then applied to the single crystal, and the temperature of the LN single crystal is decreased to a predetermined temperature of the Curie temperature or less while keeping applying the voltage, thereafter, the application of the voltage is stopped, and the LN single crystal is cooled down to room temperature. After the poling treatment, the LN single crystal (ingot), which has been abraded on its peripheral surface (cylindrically) in order to adjust the external shape of the single crystal, undergoes machining such as slicing, lapping, and polishing steps to become a wafer-shaped LN substrate. The LN substrate finally obtained is substantially colorless and transparent, and has a volume resistivity of approximately 1×1015 Ω·cm or more.
The LN substrate obtained by such a conventional method has a problem of pyroelectric breakdown in the process of manufacturing the SAW filter. The pyroelectric breakdown is a phenomenon in which due to the pyroelectric property, which is one of properties of the LN single crystal, electrical charge is charged up on the surface of the LN substrate because of the change in temperature applied by the process, and generates sparks, which cause the comb electrode formed on the surface of the LN substrate to broken, and further cause crack and the like to be generated in the LN substrate. The pyroelectric breakdown is a major factor of causing a decrease in yield in the device fabrication process. In addition, the high light transmittance of the substrate causes also a problem that light transmitted through the substrate in the photolithographic process, which is one of the device fabrication processes, is reflected at the back surface of the substrate to return the front surface, causing the resolution of the formed pattern to deteriorate.
In view of this, to solve this problem, Patent Document 1 proposes a method in which an LN substrate (wafer) which is cut out from an ingot is exposed to a chemical reducing atmosphere of a gas selected from argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, and oxygen, as well as combinations of these at a temperature within a range of 500 to 1140° C. to be blackened to lower the volume resistivity of the LN substrate, thereby reducing the pyroelectric property. Note that performing the above-described heat treatment causes the LN crystal, which has been colorless and transparent, to become colored and opaque. Since the color tone of the colored and opaque crystal then observed looks brown to black with a transmitted light, the phenomenon in which a crystal becomes colored and opaque is herein referred to as “blackening”. It is considered that the blackening phenomenon occurs because oxygen defects (voids) are introduced into the LN substrate by the reduction treatment and a color center is thus formed. It is considered that the change in volume resistivity occurs because the valence of Nb ions changes from 5+ to 4+ and free electrons emitted from Nb ions increase in the substrate to compensate for deviation of the charge balance due to the generation of oxygen defects. Accordingly, the degree of blackening and the resistivity are substantially proportional to each other.
Meanwhile, since the method described in Patent Document 1 includes heating the LN substrate to a high temperature of 500° C. or more, the treatment time is short; however, variations are likely to occur in blackening between treatment batches. In addition, color non-uniformity due to the blackening, that is, in-plane distribution of resistivity is likely to occur in the heat-treated substrate. Thus, there is a problem that a decrease in yield in the device fabrication process still cannot be sufficiently prevented.
For this reason, as a method to solve the above-described problems, Patent Document 2 proposes a method in which an LN substrate is heat-treated at a low temperature of 300° C. or more and less than 500° C. in a state where the LN substrate is buried in a powder formed of at least one element selected from the group consisting of Al, Ti, Si, Ca, Mg, and C.